Studies are in progress on the transport and metabolism of vitamin A. These studies aim to define in chemical and physical terms the transport system responsible for the transport of vitamin A in plasma, and to explore its roles in health and disease. The complete amino acid sequence of human prealbumin (PA) has been determined, and the amino acid sequence of human retinol-binding protein (RBP) is under study. Studies in experimental animals have employed the rat as an animal model to study RBP production and secretion by the liver. These studies aim to define the mechanisms which regulate RBP production in the liver, and hence control vitamin A delivery to tissues. Rat PA has been isolated and partly characterized. Although similar in their overall chemical structures, rat and human PA are immunologically distinct. Metabolic studies suggest that rat RBP and PA are independently secreted by the liver. Other studies have examined the role of vitamin A, and its metabolism and transport, during fetal development in the rat. Vitamin A-deficient rats show fetal growth failure by the 13th day of gestation. The effects of hypervitaminosis A on the metabolism of RBP and vitamin A were examined in rats. Rats given large doses of vitamin A show elevations of serum vitamin A levels (mainly increases in retinyl ester levels), and decreases in serum RBP levels. RBP may function not only to regulate the supply of retinol to tissues, but also to protect tissue membranes from the surface-active properties of the vitamin. Clinical studies have continued to examine the effects of a variety of diseases on the levels of plasma RBP and PA in humans.